ATLANTA - The Atlanta Falcons hired Bobby Petrino because they thought he could turn Michael Vick into a complete quarterback. They signed Joey Harrington merely to serve as No. 7’s backup.

My, how things have changed.

Harrington is now the starter by default, leaving Petrino and the Falcons without their most prominent player as they prepare to begin training camp Thursday in suburban Flowery Branch.

How can the Falcons possibly cope with the jarring change from Vick, who last season became the first quarterback in NFL history to rush for 1,000 yards, to Harrington, a first-round bust whose career record as a starter is 23-43?

Petrino can only talk bravely about a smooth transition.

“I know Joey has been through some hard times. He’s been hit a lot. But he’s hung in there,” the coach said. “The thing that impresses me most is his take-charge attitude.”

Vick is barred from training camp while the NFL conducts its own investigation into federal charges related to dogfighting. He wouldn’t have been at the first practice anyway; his arraignment in Richmond, Va., is set for Thursday.

The Falcons planned to give their star quarterback a four-game suspension — the maximum punishment allowed to a team — but held off at the urging of commissioner Roger Goodell, who could impose even stiffer penalties under a new player conduct policy.

The sordid affair has left Vick with an uncertain future on the football field. He seems certain to face some sort of suspension even before his trial, and the Falcons didn’t rule out the possibility of cutting him. Most seriously, he faces up to six years in prison if convicted.

With no choice but to move on, Petrino quickly anointed Harrington the starter and said the major battle of training camp will be deciding on a backup. D.J. Shockley or Chris Redman are already on the roster, and the Falcons will need to add another quarterback just to get through the routine of training camp.

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As for Harrington, he’s done little to justify being the third overall pick in the 2002 draft. He was let go by Detroit after going 18-37 as its starting quarterback. During one year in Miami, he started 11 games but lost that job, too, after putting up a passer rating of 0.0 in a game against Buffalo near the end of the season.

Also, the 28-year-old Harrington is a conventional dropback passer, providing none of the scrambling or running ability that made Vick such a unique weapon (hard to hold that against Harrington, of course, since no other quarterback has that sort of speed and quickness).

The investigation of Vick began just weeks after Harrington was signed, so he’s had all summer to prepare for the possibility of stepping in.

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“If I do get a chance,” Harrington said shortly after signing with the Falcons, “I’ll be ready to help this team.”

Of course, the Falcons didn’t give Petrino a five-year, $24 million deal because they wanted him to work with Harrington. They felt he could improve Vick’s erratic passing skills, which failed to develop in the West Coast offense of previous coach Jim Mora.

But Petrino insisted that any quarterback can thrive in his system.

“We’ve got a huge playbook,” he said. “My job is to understand what the quarterback can do well and what he can’t do well. Then we utilize the best parts of the offense. We really try to make the offense his own.”

As if they didn’t have enough problems, the Falcons revealed on the eve of camp that running back Warrick Dunn, who rushed for 1,140 yards last season, will miss up to four weeks after having back surgery last weekend. He was hurt during offseason workouts.

Dunn will likely miss all of two-a-days and the first two preseason games, but the Falcons expect him to be back for the Sept. 9 opener against Minnesota.